1. Field of the Invention
The present invention relates to a medical device for placing an intravascular stent at a preselected location within a vessel of the human body, and more particularly, relates to a catheter having a distal tip for retaining the stent in order to transport the stent to a preselected position within the vessel and a control mechanism for releasing the stent at the preselected position.
2. Description of the Prior Art
For many years flexible catheters have been used to place various devices within the vessels of the human body. Such devices include dilatation balloons, radiopaque fluids, liquid medications and various types of occlusion devices such as balloons and embolic coils. Examples of such catheter devices are disclosed in U.S. Pat. No. 5,108,407, entitled, "A Method And Apparatus For Placement Of An Embolic Coil"; U.S. Pat. No. 5,122,136, entitled, "Endovascular Electrolytically Detachable Guidewire Tip For The Electroformation Of Thrombus In Arteries, Veins, Aneurysms, Vascular Malformations And Arteriovenous Fistulas." These patents disclose devices for delivering an embolic coil to a preselected position within a vessel of the human body in order to treat aneurysms or alternatively to occlude the blood vessel at the particular location.
Stents which are placed in vessels may take the form of helically wound wire, or tubular like structures with numerous patterns defining the walls of the tubule. Examples of various stent configurations are disclosed in U.S. Pat. No. 4,512,338, entitled, "Process for Restoring Patentcy to Body Vessels"; U.S. Pat. No. 5,551,954, entitled, "Biodegradable Drug Delivery Vascular Stent"; and U.S. Pat. No. 4,994,071, entitled, "Bifurcating Stent Apparatus and Method." Stents are generally formed of materials that can retain their shape under the pulsatile flow conditions encountered when placed within the body vessel. Some materials that have been used to make stents include metals and alloys, such as, stainless steel, tantalum, tungsten and nitinol, as well as polymers such as polyvinyl alcohol (PVA), polyglycolic acid (PGA) and collagen. On occasion multiple stents are placed at a given location to provide the desired vascular support.
In the past, the deployment of stents has been accomplished by numerous techniques. One such technique used to deploy a typical wire stent uses a pusher wire to push the wire stent through the lumen of a properly positioned cannula. As the stent exits the cannula it takes a predetermined shape until completely deposited in the vessel. This procedure is usually conducted under fluoroscopic visualization, such that the movement of the stent through the vasculature can be monitored. With these placements systems there is very little control over the exact placement of the stent since the stent may be ejected to a position some distance beyond the end of the cannula. As is apparent, with these latter systems, when the stent has been released from the cannula it is difficult, if not impossible, to retrieve the stent or to reposition the stent.
Numerous procedures have been developed to enable more accurate positioning of stents within a vessel. One such procedure utilizes a helically wound wire loop stent with a relaxed diameter. The stent is wound on a smaller diameter delivery while fixing the ends of the stent. This keeps the stent in a small diameter, tightly wound coil. This system is then delivered through the lumen of a properly positioned catheter exiting at a desired location. Once the delivery wire is activated to release the ends of the stent, the stent radially expands to its relaxed larger diameter. Such a stent positioning method is disclosed in U.S. Pat. No. 5,772,668, entitled, "Apparatus for Placing an Endoprosthesis."
Another stent positioning system utilizes a self expanding tubular stent. This stent has a relaxed diameter that approximates the diameter of the vessel to be supported. For transport through the catheter, the stent is positioned on a smaller diameter delivery wire. A sheath is positioned over the stent/delivery wire assembly constraining the stent to a smaller diameter. Once the assembly is placed at the desired location in the vasculature, the sheath is withdrawn exposing the stent allowing the stent to return to its predetermined larger size. The expansion of the stent uncouples the stent from the delivery wire while depositing the stent in the vessel at the desired location.
Another stent positioning system utilizes a radially expandable tubular stent formed of a malleable material. This tubular stent has a predetermined expanded diameter defining a lumen that is approximately the same diameter as the vessel to which the stent will be placed. A balloon catheter is placed within the lumen of the stent and the stent is subsequently compressed to a smaller diameter on top of the balloon portion of the catheter. The assembly is then placed within a properly positioned catheter and delivered to the desired location. Inflating the balloon thereby expanding the diameter of the compressed stent deploys the stent. Once the stent is expanded to its predetermined diameter the balloon is deflated and removed leaving the stent deposited at the desired location.